As automated assembly equipment and robots become more sophisticated, it is desirable to add the sense of vision to such devices. This feature would enable the manufacturing equipment to locate the position of the object being fabricated as well as inspect the object for the presence and the proper location of various components.
Present vision systems use a video camera to create an electronic image of the object. A technique that is often employed to detect the features of the object relies on the contrast between various portions of the image. For example if a brightly lit object is placed in front of a relatively dark background, the shape of the object may be determined by detecting points of sharp brightness contrast in the video image. Similarly the size, shape and position of holes through the object may be determined. To carry out this technique the brightness or luminance of each picture element (pixel) in the image is compared to a predetermined threshold. Those pixels falling on one side or the other of the threshold represent a given feature of the object.
A problem encountered with this approach is that variation of the object's overall illumination can change the relationship of the feature's brightness to the preset threshold. If the overall illumination changes significantly a feature may no longer be brighter than the given threshold value, for example. In some applications the illumination can be tightly controlled. However, in object recognition systems used in an industrial environment, the amount of light falling on the objects being sensed may vary widely. Other objects passing nearby may partially block some of the illumination and various manufacturing processes such as arc welding may increase the light levels.
In television broadcast situations where the light levels cannot be closely regulated, such as outdoor broadcasts, a video engineer continuously monitors and normalizes the video levels from the various cameras. In an automated vision system it is self defeating to provide a human operator to continuously monitor the video signal and compensate for variations in the illumination of the object.